Condensers and radiator heat exchangers for automotive applications are often painted black in order to reduce their metallic visibility through the front grill of an automobile. While various paints and painting processes have been developed to enhance the quality of the paint and achieve a more efficient and cost effective painting process, a significant disadvantage is the volatile emissions that are inherent with the use of paints. In addition, a significant amount of paint waste is typical in any painting process. Accordingly, alternatives to painting such components would be desirable.
A black oxide layer can typically be formed on aluminum and its alloys by first anodizing the metal surface to form an aluminum oxide (alumina) layer. This anodic oxidation process is performed in an electrolyte solution that typically contains sulfuric, chromic or oxalic acids, and converts the aluminum at the metal surface to alumina. The alumina layer must then be treated with an appropriate solution to generate the desired black coloration. While black oxide coatings are widely used in various applications, they generally have not been applied to heat exchanger assemblies due to the requirement for the anodizing process. In particular, anodizing of a heat exchanger is expensive due to the heat exchanger's large surface area. Furthermore, a uniform anodized oxide layer cannot be easily formed on a heat exchanger due to its compactness.
To overcome the above, various solutions have been suggested in the prior art to blacken an alumina layer on the surface of an aluminum alloy without the requirement for an anodization step. One such solution has been a mixture of copper nitrate and potassium permanganate. However, desirable results have not been readily obtainable with this solution, and the presence of copper in this solution is detrimental to the corrosion resistance of aluminum alloys, particularly those of the type used to form heat exchangers.
In view of the above, it is apparent that an alternative to painting a heat exchanger would be desirable. However, it is also apparent that a black oxide coating capable of providing the desired black coloration for automotive heat exchangers has not been achieved to date, as a result of required additional processing steps or the use of solutions that are not compatible with large-scale production practices.